In certain classes of hauling operations, such as moving harvested trees from their felling point to a collection point, there is often no feasible means of transporting a load other than by dragging it behind a vehicle. This is generally accomplished either by attaching the load to a load skidding vehicle with a cable or by grasping the load with a grapple suspended from an elevated support boom borne by the vehicle. Basic examples of grapple assemblies for use in load skidding applications are set forth in U.S. Pat. No. 3,620,394 which issued to Symons et al. on Nov. 16, 1971 and U.S. Pat. No. 3,513,998 which issued to Stone et al. on May 26, 1970.
The prime disadvantage of existing grapple skidders as opposed to cable skidders is that design strictures of the former necessitate that the grapple be pivotally connected to its support boom at a pivot point substantially above and behind the rear axle of the skidder. This results in the load being borne by the vehicle at this pivot point, this being the first point through which the load passes which is also rigidly connected to the skidding vehicle. This loading imposed upon the skidding vehicle may be described by a load vector having essentially two components both of which pass through the support boom-grapple pivot point. The first, rearwardly directed from and parallel to the longitudinal axis of the skidding vehicle, is the force required to overcome friction between the load and the ground across which it is skidded. The second, vertical and downwardly directed, is that portion of the weight of the load borne by the grapple.
As this load vector passes through a rearwardly extended elevated position, a substantial overturning moment exists about the rearmost ground contacting point of a loaded skidding vehicle. Consequently, current grapple skidders lose maneuverability when skidding a load. For those grapple skidders including a plurality of axles, the rearmost axle will suffer disproportionately great loading. Moreover, aside from the reduction in maneuverability there is a relatively low limit on the size of the load that can be carried with a grapple skidder. In some instances a load easily handled by a cable skidder will, if attempted with a grapple skidder of equivalent power, lift the front of the grapple skidder from the ground.
Efforts have previously been made toward the solution of these problems. One scheme involves the use of a force applying arm attached to the vehicle for applying a downward force on the load at a point rearwardly located from the grapple. Activating the arm provides a reactive force tending to equalize the weight distribution on the axles of the vehicle. This development is disclosed in U.S. Pat. No. 4,140,233 which issued to Muntjanoff et al. on Feb. 20, 1979.
Another manner of mitigating the problems detailed above involves fastening a cable from the boom supported grapple to the vehicle such that the cable may be placed in tension under load conditions and made slack under no load conditions. This results in the load being primarily carried by the cable thereby lowering the load vector and consequently decreasing the distance or moment arm from the rearmost point of skidder-ground contact to the load vector. Decreasing the moment arm decreases the overturning moment resulting in increased stability. This scheme is described in U.S. Pat. No. 3,746,193 which issued to Eaves on July 17, 1973. Among the disadvantages of this advance are the need to include a winch on the skidding vehicle and the difficulties presented in achieving free rotation of the grapple about its vertical axis owing to the attached cable.
Another problem that is common in conventional grapple type load skidding vehicles is that they permit free swinging movement of the grapple when the vehicle is travelling without a load. This movement of the grapple about its pivot point is often severe enough that the grapple comes in contact with some portion of the vehicle, occasionally resulting in damage. A means of preventing this involves snubbing the grapple to the back of the skidding vehicle by means of a cable and winch assembly. Such a scheme is set forth in U.S. Pat. No. 3,907,137 issued to Korbel et al. on Sept. 23, 1975. The significant disadvantages of this development are the necessity for a winch and a potentially obstructing cable.
Primary objectives for the design of conventional grapple type log skidding vehicles include minimizing the weight of the grapple and its supporting structure so as to maximize the vehicle's log payload and minimizing the height above the ground for the grapple and its supporting structure's center of gravity so as to maximize the skidder's stability for the positions assumable by such grapple and supporting structure. A further objective is to minimize the number of pin joints due to their expense and service requirements. U.S. Pat. No. 4,005,894 which issued Feb. 1, 1977, illustrates a log loader apparatus which includes a manipulatable, multi-boom grapple. The fluid ram for manipulating the lower boom is Joined at one end to the loader's turntable and the fluid ram for manipulating the upper boom is joined at one end to the lower boom. Such elevated disposition of the rams contribute to the vehicle's instability.
Such log loader is stationary during operation of its turntable and multiple booms and thus permits use of the illustrated outriggers to provide stability for it. Moreover, the fluid ram connected between the booms exerts bending stresses on the lower boom during operation which either decreases the life of that boom or necessitates the use of a heavier lower boom which results in a commensurately smaller workload.
In addition to the characteristics of reduced stability and smaller payload such apparatus would contribute to a skidder vehicle, a further disadvantage of such elevated boom manipulating fluid rams is the additional length of fluid lines needed as well as the extra shielding and associated weight of same required to protect the lines from hazards which are commonplace in a woods environment. Additionally, the extra pin joints and associated linkage between the grapple and elevated grapple tilting cylinder increase the apparatus' expense, further reduce its stability, and decrease the reliability of the utilizing vehicle.
The present invention is directed to overcoming one or more of the problems as set forth above.